Diagnostic devices are known which are able to give a quick, direct reading analysis of a blood or other body fluid sample for its content of a specific substance, such as sugar, alcohol, cholesterol, drugs, etc. These devices make use of a test strip carrying a composition (usually a conductive enzyme composition) which, when wetted by the body fluid sample, reacts electrochemically with the predetermined substance to be detected in the sample to provide an electrical signal which is measured and analysed by an electronic detector part of the device and displayed as a calibrated read-out. Test strips are also known in which the active composition is designed to change colour according to the content of the substance to be detected in the sample. In some cases the test strip may be dipped directly into the fluid sample to be tested, but generally it is more usual to apply a droplet of the sample onto a designated target area of the strip.
These devices are generally quite accurate if used correctly and under ideal conditions, but can all too easily give a false reading. The most common causes of error are excessive wetting of the test strip leading to electrode flooding, and excessive evaporation from the sample during the test reaction time (which may take up to 60 seconds) which effectively increases the measured concentration of the detected substance. In some cases the presence of red blood cells or other protein cells can also affect the reaction and hence the accuracy of the test result.
It has therefore been proposed to cover the target area of the test strip by a semi-permeable membrane so that the fluid sample is applied to the target area in a controlled manner through the membrane, the membrane also acting to allow efficient oxygen permeation through the membrane to the target area while controlling the rate of evaporation from the sample applied to the target area.
Microporous PTFE membranes, such as GORE-TEX (R.T.M.) membranes manufactured in accordance with British Patent No. 1355373 of W. L. Gore & Associates Inc., are potentially very suitable for this purpose because of their high porosity and relatively narrow pore size distribution, and also because they are strong, chemically inert and steam sterilizable. Unfortunately, they cannot be used in their natural state because they are highly hydrophobic, thus inhibiting the transmission of aqueous fluids, and they are also protein repellent which, in the case of blood samples, can lead to premature coagulation and consequent blinding of the membrane.
Several treatments are commonly used for inducing hydrophilic activity in naturally hydrophobic polymeric membranes, but generally these use detergent surfactants based on metal soaps, fatty acid esters, ammonium salts or amines etc., and are unsuitable for membranes intended for use on diagnostic test strips as described above.
It is also known that a porous PTFE membrane can be rendered hydrophilic by impregnating it with a hydrophilic, water soluble polymer, such as polyvinylacohol, and then fixing the polymer by rendering it water insoluble by cross-linking. We have found that such hydrophilic polymer coatings applied to the pores of a microporous PTFE membrane can also be fixed by using a suitable complexing agent which forms a complex with the polymer, and that the complex exhibits good protein bonding characteristics in the presence of water.